This invention relates to wireless communication systems, and more particularly to the accelerated selection of a base station by a digital wireless transceiver.
Upon powering up, a mobile wireless transceiver, or mobile station, must establish communication with a base station in a wireless network before the mobile station can begin communicating over the wireless network. The mobile station usually searches for the nearest base station, or the station with the strongest signal, to ensure quality communication.
Many digital wireless units, such as CDMA handsets, rely on a digital spreading sequence to spread transmitted signals over a wide range of frequencies, a technique commonly known as direct sequence spread spectrum (DSSS) communication. In general, the spreading sequence is a series of binary units, or chips, that repeats itself with period T. The spreading sequence is produced by a pseudo random sequence generator, such as a linear feedback shift register.
In a typical DSSS system, such as a CDMA network, each base station transmits a pilot signal generated by digitally spreading a constant sequence (e.g., all zeroes). In general, one spreading sequence is used for all base stations in a network, but a timing offset exists between the base stations. As a result, the spreading sequences of the various base stations are not time-aligned, even though their time evolution is identical.
To establish communication with the nearest base station, a mobile unit first must detect the base station""s pilot signal. This process requires time aligning the spreading sequence generated in the mobile unit with the spreading sequence generated in the base station (known as xe2x80x9cdespreadingxe2x80x9d). In general, the mobile unit does this by searching for signals at all possible code phases (initial conditions or time offsets) of the spreading sequence and then aligning itself with the code phase associated with the base station producing the strongest signal.
By way of example, a typical wireless CDMA network uses a spreading sequence that repeats itself every 215 chips, and thus has 215 distinct code phases. To ensure fine enough sampling, {fraction (1/2+L )}-chip code phase sampling is usually required. In IS-95, the chip rate is approximately 1.23 MHZ. For this type of system, an exhaustive search to discover the best {fraction (1/2+L )}-chip sample code phase could take as long as approximately 3.4 seconds to complete. Other factors often lengthen this delay. Most wireless customers are not willing to tolerate such long delays in service.
Many mobile stations reduce search time by using several correlators to search several code phases at once, a technique that increases a unit""s power consumption over the course of the search. Thus, while the delay in service at start-up is reduced, the mobile unit""s battery life also is reduced because it must power more circuitry.
The invention, in its various aspects and several embodiments, reduces the search-and-acquisition time and power consumption of a wireless mobile unit during the unit""s search for a nearby base station, such as a CDMA base station. In many cases, the invention also improves xe2x80x9chard-handoverxe2x80x9d of an established communication as the mobile unit moves from one wireless network to another (e.g., from an AMPS network to a CDMA network). In most situations search time is reduced by at least one-half, and it frequently is improved by as much as thirty-fold, without the addition of additional signal processing circuitry. As a result, wireless customers experience a shorter delay in service at power-up without a reduction in battery life.
In certain aspects, the invention involves selecting one of multiple nearby base stations to facilitate communication over a wireless network. Each base station transmits a pilot signal that includes a spreading signal transmitted at one of many possible phase offsets. A wireless receiver implementing the invention selects an initial one of the possible phase offsets of the spreading signal and then correlates a received signal against the spreading signal at consecutive ones of the possible phase offsets. Correlation begins at the selected initial phase offset and continues until a component of the received signal and the spreading signal are aligned, indicating the detection of the pilot signal from one of the base stations. The receiver then selects a non-consecutive one of the possible phase offsets of the spreading signal and correlates the received signal against the spreading signal at this non-consecutive phase offset to determine whether any other component of the received signal is aligned with the spreading signal at the selected non-consecutive phase offset.
In certain embodiments, if no component of the received signal is aligned with the spreading signal at the selected non-consecutive phase offset, the receiver correlates the received signal against consecutive ones of the possible phase offsets, beginning at the selected non-consecutive phase offset, until another component of the received signal is aligned with the spreading signal. The receiver then selects another non-consecutive one of the possible code phases at which to correlate the spreading signal against the received signal. In general, the selected initial phase offset and the selected non-consecutive phase offsets are separated from each other by a predetermined number of possible phase offsets. In many systems, such as CDMA networks, the possible phase offsets are equally spaced.
In some embodiments, the receiver selects the base station that produces the pilot signal having the greatest received signal strength and ignores any pilot signal with a received signal strength that is below a predetermined threshold. In other embodiments, selecting a non-consecutive one of the possible phase offsets includes selecting a phase offset at which a pilot signal is expected to occur and defining an aperture that begins a predetermined number of chips of the spreading signal before the selected phase offset and ends a predetermined number of chips of the spreading signal after the selected phase offset. Some embodiments use an aperture that has equal numbers of chips before and after the selected phase offset, and other embodiments use an aperture that has unequal numbers of chips before and after the selected phase offset. The signals are correlated across the aperture to allow for propagation delay in the received signal.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.